This binder is preferably a thermoplastic resin and examples thereof include homopolymers and copolymers of acrylic monomers such as acrylic acid, methacrylic acid, acrylic acid ester and methacrylic acid ester; cellulose-base polymers such asmethyl cellulose, ethyl cellulose and cellulose acetate; homopolymers and copolymers of vinyl-base monomers, such as polystyrene, polyvinylpyrrolidone, polyv